A person that is morbidly obese is prone to many health problems. They will have to pay more co-pays/deductibles, etc. to receive the extra care they need under insurance. So, if they require two seats on an airplane, they should pay for two seats.
The space allowed for passengers has shrunk over the past years, yet people are getting larger. I tend to be a bit claustaphobic and the idea of someone who needs more than one seat "sharing" part of mine is very uncomfortable.
Obesity is not a moral issue. There are multiple factors involved, some beyond the control of the ones who suffer from obesity. Still, it is only fair and kind for them to pay for two seats if they are using two seats.